The switch from fetal to adult hemoglobin in humans is regulated by transcription factors that enhance or inhibit interactions of the gamma- or beta-globin gene with the powerful Locus Control Region. We recently demonstrated that knockdowns of KLF1 in human and mouse adult erythroid progenitors reduce BCL11A levels and increase human gamma- to beta-globin gene expression ratios. These results suggest that KLF1 controls globin gene switching by directly activating beta-globin and indirectly repressing gamma-globin gene expression. In addition, we have recently isolated a protein (FOKLF1; Friend of KLF1) that complexes with KLF1 and activates beta-globin gene expression. Interestingly, knockdowns of FOKLF1 dramatically increase human gamma- to beta-globin gene ratios. These results suggest that controlled knockdown or pharmacological inhibition of FOKLF1 in adult erythroid progenitors may provide a method for activating fetal hemoglobin in patients with beta-thalassemia and sickle cell disease. The Specific Aims of our proposal are as follows: (1) To define FOKLF1 domains essential for gamma- to beta-globin genes witching (2) To define FOKLF1 and KLF1 gene regulatory elements responsible for the up-regulation of these genes during development. (3) To identify HPFH (Hereditary Persistence of Fetal Hemoglobin) individuals who have mutations/variations in the FOKLF1 gene.